Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Nanoparticles: Properties, applications and toxicities

The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

Semiconductor photocatalysis is recognized as a promising strategy to simultaneously address energy needs and environmental pollution. Titanium dioxide (TiO2 ) has been investigated for such applications due to its low cost, nontoxicity, and high chemical stability. However, pristine TiO2 still suffers from low utilization of visible light and high photogenerated-charge-carrier recombination rate. Recently, TiO2 photocatalysts modified by dual cocatalysts with different functions have attracted much attention due to the extended light absorption, enhanced reactant adsorption, and promoted charge-carrier-separation efficiency granted by various cocatalysts. Recent progress on the component and structural design of dual cocatalysts in TiO2 photocatalysts is summarized. Depending on their components, dual cocatalysts decorated on TiO2 photocatalysts can be divided into the following categories: bimetallic cocatalysts, metal-metal oxide/sulfide cocatalysts, metal-graphene cocatalysts, and metal oxide/sulfide-graphene cocatalysts. Depending on their architecture, they can be categorized into randomly deposited binary cocatalysts, facet-dependent selective-deposition binary cocatalysts, and core-shell structural binary cocatalysts. Concluding perspectives on the challenges and opportunities for the further exploration of dual cocatalyst-modified TiO2 photocatalysts are presented.

Dual Cocatalysts in TiO 2 Photocatalysis.

Graphitic carbon nitride (g-C3N4)-based photocatalysts holds great promise on photocatalytic CO2 conversion into solar fules; however, the efficiency of pristine g-C3N4 is currently limited by its poor visible light absorption and rapid charge recombination. Employing silver chromate (Ag2CrO4) nanoparticles as photosensitizer and graphene oxide (GO) as cocatalyst, a novel ternary Ag2CrO4/g-C3N4/GO composite photocatalyst was fabricated for photocatalytic CO2 reduction into methanol (CH3OH) and methane (CH4). The ternary composites exhibited an enhanced CO2 conversion activity with a turnover frequency of 0.30 h–1, which was 2.3 times that of pristine g-C3N4 under simulated sunlight irradiation. The enhanced photocatalytic activity was due to broadened light absorption, higher CO2 adsorption and more efficient charge separation. Specifically, due to the matched band structure and appropriate loading ratio of Ag2CrO4, a direct Z-scheme Ag2CrO4/g-C3N4 heterojunction is formed, driven by the internal electric field across the Ag2CrO4/g-C3N4 interface. The formation of the direct Z-scheme heterojunction is substantiated by radical scavenging experiments and density functional theory calculations, and it benefits the photocatalytic reaction by accelerating the charge separation and improving the redox ability. Furthermore, GO cocatalyst not only promotes the charge transfer but also provides plentiful CO2 adsorption and catalytic sites. This work exemplifies the facile development of ternary g-C3N4-based photocatalysts with high CO2-conversion activity by coupling a small amount of Ag-based photosensitizer and metal-free cocatalyst.

Ag2CrO4/g-C3N4/graphene oxide ternary nanocomposite Z-scheme photocatalyst with enhanced CO2 reduction activity

The high dispersibility and solubility are highly required for the potential applications and development of well-known g-C3N4 material. In this study, a facile hydrothermal treatment and the following vacuum freezing-drying process was developed to synthesize the g-C3N4 nanosheets (ca. 5 nm) with excellent dispersibility and solubility in aqueous solutions. It was found that the melem structures with many hydrophilic groups ( NH2, −OH and C O) were formed on the g-C3N4 nanosheet surface, resulting in the formation of soluble g-C3N4 (SCN) nanosheets. Moreover, the SCN nanosheets can be worked as the effective modifier to greatly increase the H2-production performance of conventional g-C3N4 photocatalyst (the resultant sample was referred to SCN/g-C3N4). Photocatalytic results revealed that the SCN/g-C3N4 sample exhibited a remarkably higher H2-production performance than the pure g-C3N4 by a factor of ca. 2. The improved H2-production rate of SCN/g-C3N4 photocatalysts could be primarily ascribed to the introduction of hydrophilic groups, which not only remarkably enhances the dispersibility and hydrophilicity of SCN/g-C3N4, but also work as the interfacial active sites to accelerate the H+-reduction reaction and the rapid formation of H2. The present soluble g-C3N4 nanosheets provide potential various applications in environmental protection and energy conversion fields.

Soluble g-C3N4 nanosheets: Facile synthesis and application in photocatalytic hydrogen evolution

For noble metal cocatalyst-modified photocatalysts with a high H 2 -evolution performance, both of the rapid electron transfer (or electron capture) from semiconductor to cocatalyst and the following interfacial catalysis for H 2 production on the cocatalyst surface are highly required. Compared with Pt-loaded photocatalyst, Au-modified sample usually shows an obviously lower H 2 -evolution activity due to its inefficient H 2 -production rate on the Au cocatalyst surface. In this study, the SCN − ions as the catalytic active sites are selectively adsorbed on the Au surface of CdS/Au photocatalysts (CdS/Au-SCN), with the aim of promoting the interfacial H 2 -evolution reaction on Au cocatalyst and improving the photocatalytic water-splitting efficiency of CdS/Au system. The CdS/Au-SCN photocatalysts were synthesized via a two-step process including the initial photoinduced deposition of Au nanoparticles on CdS surface and the following selective adsorption of SCN − on the Au surface by an impregnation method. It was found that the resultant CdS/Au-SCN photocatalysts exhibited obviously improved photocatalytic H 2 -evolution activity compared with the CdS, CdS/Au and CdS/SCN photocatalysts. Especially, the CdS/Au-SCN(0.5 mM) achieved the highest photocatalytic activity (109.60 μmol h −1 ) with an apparent quantum efficiency (QE) of 11.25%, which was clearly higher than that of CdS and CdS/Au by a factor of 2.3 and 1.5 times, respectively. The enhanced H 2 -evolution performance of CdS/Au-SCN can be attributed to the excellent synergistic effect of Au and SCN − , namely, the Au nanoparticle functions as an effective electron-transfer mediator for the steady capture and rapid transportation of photogenerated electrons from CdS surface, while the adsorbed SCN − serves as the interfacial catalytic active-site to effectively adsorb H + ions from solution and promote the rapid H 2 -evolution reaction. The present synergistic mechanism of electron-transfer mediator and interfacial catalytic active-sites may provide some new insights for the design of highly efficient photocatalytic materials.

Synergistic effect of electron-transfer mediator and interfacial catalytic active-site for the enhanced H2-evolution performance: A case study of CdS-Au photocatalyst

Metallic Ag has been widely demonstrated to be an excellent oxygen-reduction cocatalyst to significantly improve the photocatalytic decomposition performance of various organic substances. However, as a H2-evolution cocatalyst, the improved photocatalytic performance by metallic Ag is quite limited due to its low H2-evolution rate. In this study, for the well-known TiO2/Ag photocatalyst, Ag2S as the efficient H2-evolution active sites was selectively loaded on the metallic Ag surface to greatly promote the interfacial H2-evolution reaction rate. In this case, the TiO2/Ag-Ag2S sample was synthesized by a two-step process including the simple photoinduced deposition of metallic Ag on the TiO2 surface and the following in situ sulfidation of partial Ag into Ag2S. Photocatalytic experimental results indicated that the TiO2/Ag-Ag2S(40uL) photocatalysts clearly exhibited a significantly higher UV-light photocatalytic H2-evolution activity (119.11 μmol h−1) than the pure TiO2, TiO2/Ag and TiO2/Ag2S photocatalysts by a factor of 51.8, 3.9 and 3.6 times, respectively. On the basis of the present results, a synergistic effect of dual electron-cocatalyst (metallic Ag and Ag2S) is proposed for the improved photocatalytic H2-evolution activity, namely, the Ag-nanoparticle cocatalyst can steadily capture and transfer the photogenerated electrons from TiO2 surface, while the Ag2S cocatalyst is considered to be the interfacial active sites to promote the rapid H2-evolution reaction. This research may provide new strategies for the development of highly efficient photocatalytic materials used in various fields.

Promoting the interfacial H2-evolution reaction of metallic Ag by Ag2S cocatalyst: A case study of TiO2/Ag-Ag2S photocatalyst

The preferential adsorption of targeted contaminants on a photocatalyst surface is highly required to realize its photocatalytic selective decomposition in a complex system. To realize the tunable preferential adsorption, altering the surface charge or polarity property of photocatalysts has widely been reported. However, it is quite difficult for a modified photocatalyst to realize the simultaneously preferential adsorption for both cationic and anionic dyes. In this study, to realize the selective adsorption for both cationic and anionic dyes on a photocatalyst surface, the negative reduced graphene oxide (rGO) nanosheets and positive phenylamine (PhNH2) molecules are successfully loaded on the TiO2 surface (PhNH2/rGO-TiO2) with spatially separated adsorption sites, where the negative rGO and positive PhNH2 molecules work as the preferential adsorption sites for cationic and anionic dyes, respectively. It was interesting to find that although all the TiO2 samples (including the naked TiO2, PhNH2/TiO2, r...

Fazhou Wang

Papers

Soluble g-C3N4 nanosheets: Facile synthesis and application in photocatalytic hydrogen evolution

Synergistic effect of electron-transfer mediator and interfacial catalytic active-site for the enhanced H2-evolution performance: A case study of CdS-Au photocatalyst

Promoting the interfacial H2-evolution reaction of metallic Ag by Ag2S cocatalyst: A case study of TiO2/Ag-Ag2S photocatalyst

Phenylamine-Functionalized rGO/TiO2 Photocatalysts: Spatially Separated Adsorption Sites and Tunable Photocatalytic Selectivity.

The effect of curing regimes on the mechanical properties, nano-mechanical properties and microstructure of ultra-high performance concrete